Butterfly valves have been around a long time. One can find butterfly valves in numerous applications such as sewer vent stack checks, turbine checks, exhaust brakes, throttle valves, and air intake shut-off devices. Simplicity of operation make the butterfly valve the choice in these and many other applications. The offset butterfly valve was developed to satisfy the need for self closure which works nicely for check valves of all sort. One of the problems with the offset butterfly, is that rotating a round or oblique valve plate in a cylindrical conduit about a rotational axis offset from the center of the valve plate and offset from the center of the conduit is impossible because the maximum diameter of the valve plate will not be accommodated by the dimensional cross section within the conduit along the plane parallel with the rotation axis of the valve plate and parallel with the center of the conduit. An assortment of solutions to this problem have been described in the prior art, such as:
Biddle, U.S. Pat. No. 3,941,151 which describes and claims a vent pipe check valve with offset valve plate that provides an example of placing semi-annular seat means for sealing at upper and lower surfaces of the valve plate on respective opposite sides of the valve plate pivot axis.
Meneely, U.S. Pat. No. 5,676,110 which describes and claims an exhaust brake with offset butterfly valve plate that provides an example of enlarging an upstream section of the conduit to accommodate the maximum diameter of the valve plate rotating into the enlarged area.
Herron, U.S. Pat. No. 5,392,812 which describes an offset hinged flapper valve useful as a turbine inlet check valve providing an example of altering the valve plate dimensions with side planar surfaces, which decrease the maximum diameter of the valve plate, while also providing accompanying planar surfaces on the interior of the conduit to accommodate the valve plate.
While the prior art has satisfied numerous problems associated with offset butterfly valves there is always a need to improve air flow efficiency through the valve air flow conduit which, as those skilled in the art know, is accomplished by reducing air flow disruption caused by interruptions in the air flow path. The present invention accomplishes this by providing a butterfly valve with no interruptions extending into the air flow conduit, besides the valve plate, while also providing an air flow conduit with an essentially round cross section with a diameter which may be maintained throughout the length of the valve and while the valve plate is dimensioned as an oblique cross section of the conduit. The axis of rotation of the valve plate is offset from the center of the valve plate and further offset from the center of the conduit while rotation of the valve plate within the conduit to an open position where the valve plate is aligned with a plane parallel with the axis of rotation of the valve plate and the center of the conduit is accomplished by providing two opposing indentations on the interior surface of the conduit which accommodate the maximum diameter of the valve plate when rotated to an open position.